It is known in the art of medical imaging and therapy that ultrasonic energy can be used for both diagnostic purposes and therapeutic purposes. For example, high-intensity focused ultrasound (HIFU) beams can be used to treat tumors by causing local focal temperature increases that cause cell necrosis. In using HIFU, the location of the focused ultrasound beam must be determined to place the beams focal point on the tissue (tumor) which is targeted for therapy. In addition, it is desirable to sense and monitor changes which are induced by the HIFU beam within the exposed tissue.
It is known that non-linear propagation occurs in a medium, such as tissue, which is exposed to intense ultrasound pressure, such as that which occurs from a HIFU beam. The HIFU beam is a high intensity pressure wave which alternately compresses and relaxes the tissue during a signal cycle. As a beam propagates, regions of compression can disturb local propagation speeds and result in regions of increased speeds in compression segments and decreased propagation speeds in rarefaction segments of the wave. This effect locally increases with increasing peak pressure values and also exhibits a cumulative nature, i.e., becoming more prominent as an intense beam propagates further into a medium. This tends to distort the propagating pressure wave and enhance non-linearities in the echo signal. This results in a generation of higher-order harmonics and mixing products in a propagating ultrasound signal. This process is altered, however, by attenuation losses in tissue, which typically increase with increasing frequency.
The use of harmonic imaging in diagnostic imaging is also known in the prior art. However, such systems have been generally used to image non-linear scattering from small, gas-filled contrast agent particles. An example of this can be found in applicants' copending application Ser. No. 09/318,882, filed on May 26, 1999 and entitled, “Ultrasonic Systems and Methods for Fluid Perfusion and Flow Rate Measurement,” which is hereby incorporated by reference. There are also reports that different tissues exhibit different non-linear properties that can be observed in second-harmonic images if the peak pressures of the launched pulse are sufficiently large.
Although harmonic imaging techniques have been evaluated and the non-linear properties of tissues have been observed in the past, these effects have not been used in an advantageous matter to guide and monitor the progress of therapeutic ultrasound.